1. Field of the Invention
The invention relates generally to sensing assemblies. More particularly, the invention relates to sensing assemblies including giant magnetoresistors with reduced hysteresis and high sensitivity.
2. Description of the Related Art
Various types of non-contact measuring device are presently available for measuring linear and angular displacement and detecting absolute position of an object in linear or angular system along with speed. A classic example is a sliding resistor potentiometer which, as known to those skilled in the art, is generally unreliable. Optical position devices are also available which utilize optical sensors to provide an optical scale such as a slit scale. While generally more reliable than the aforementioned resistor potentiometer instruments, optical position devices nonetheless require complicated construction and are, thus, expensive to manufacture and somewhat difficult to use. They are also very sensitive to dirt and other contaminates and have been found in practice very difficult to keep clean.
There are also available magnetic scales wherein a scale written into a magnetic medium is read out by a magnetic sensor. Again, however, these devices require a fairly complicated structure and are, thus, expensive to manufacture and difficult to use.
It is well established that giant magnetoresistive properties occur with structures in which thin films of a ferromagnetic metal are separated by thin films of a non-ferromagnetic metal. A well-known example uses thin films of cobalt separated by thin films of copper. The thickness of the non-ferromagnetic separator should be chosen to promote antiferromagnetic coupling. For example, this is achieved with cobalt films and copper separator thicknesses of approximately 9 Angstrom and approximately 20 Angstrom, respectively. Typical ferromagnetic thicknesses are of the order of 10 Angstrom. For convenience, we shall refer to such films as "conventional" ferromagnetic films. Typically, these devices contain 20-30 ferromagnetic films with a corresponding number of non-ferromagnetic separators, but the minimum workable device would contain two ferromagnetic films and one non-ferromagnetic separator. All such devices require a suitable substrate, on which the stack of ferromagnetic and non-ferromagnetic films is deposited, and they may also contain cap films on top of and buffer films under the stack.
One attempt to overcome the aforementioned difficulties associated with the prior art devices is disclosed in U.S. Pat. No. 5,313,186, issued to Schuhl et al. on May 17, 1994. The Schuhl et al. reference discloses a sensor used to sense weak magnetic fields. The sensor incorporates a giant magnetoresistive device. More specifically, the giant magnetoresistive device disclosed in the Schuhl et al. reference includes a metallic multilayered structure formed by alternating magnetic and non-magnetic metals. The magnetic films all have the same thickness. The non-magnetic metals are disclosed having equal thickness and, in other embodiments, varying thicknesses. The thicknesses of the ferromagnetic films vary between five Angstrom to 100 Angstrom such that, when there is no magnetic field, the films of the magnetic material have an anti-ferromagnetic type of coupling. The differences in thicknesses of the non-magnetic materials results in coupling or the frustration thereof with respect to the magnetic films.
Although the structure disclosed in the Schuhl et al. reference is highly sensitive to weak magnetic fields, the structure is an impractical sensor in environments wherein absolute position is required. More specifically, the use of a giant magnetoresistive sensor produces hysteresis. The resistance maximum does not occur exactly at zero applied field. Instead, the resistance lags behind the applied field as the magnetic field is cycled between extreme positive and negative values. This causes an undesirable uncertainty in the magnetic field that is associated with a particular value of the resistance. The occurrence of hysteresis makes the giant magnetoresistor sensor disclosed in the Schuhl et al. reference undesirable for applications where the sequence of previous positions is arbitrary, rather than well defined.